1. Field
The current invention is in the field of metrology of objects. Particularly, the invention relates to metrology of angles, quality, and anomalies in a cone.
2. Related Art
Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head “flies” over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density of media has tended to increase and the size of storage systems has tended to decrease. This trend has led to a need for greater precision, which has resulted in tighter tolerancing for components used in disc drives. In turn, achieving tighter tolerances in components requires increased precision in metrology systems for characterizing and parameterizing those components. Measuring angles of objects is one aspect of metrology, and measuring angles of conical cavities is of interest for some disc drive designs.
Metrology systems may include systems that use technology requiring contact with a workpiece as well as systems that obtain metrology data without contacting a workpiece. It is often the case that non-contact systems can be more precise than contact systems, but can be more expensive.
U.S. Pat. No. 7,350,308 (“the '308 patent”), herein incorporated by reference in its entirety, is an exemplary system used for measuring the angle of conical cavities. The system uses a two sphere method to determine the each cone's characteristics. FIG. 1 illustrates aspects of the conceptual two sphere method for deriving an angle 2θ 114 of a conical cavity 108 (shown in cross-section), that may exist for example in a conical bearing sleeve. A first sphere 112 having a known (or determinable) diameter is inserted in the conical cavity 108. A first height 104 associated with positioning of the first sphere 112 is measured. This measurement may be with respect to reference 102. The first sphere 112 may then be removed from conical cavity 108. A second sphere 110 is inserted into the conical cavity 108. A second height 106 associated with positioning of the second sphere 110 is measured; second height 106 may also be a measurement with respect to the reference 102. After obtaining the first height 104 and the second height 106, an angle equal to one half the angle 2θ 114 may be calculated by application of the formula below, where R1, H1, R2, and H2 respectively refer to the radius of the first sphere 112, the first height 104, the second sphere 110, and the second height 106.
  θ  =      α    ⁢                  ⁢                  sin        ⁡                  [                                    (                                                R                  1                                -                                  R                  2                                            )                                                      (                                                      H                    2                                    -                                      H                    1                                                  )                            -                              (                                                      R                    1                                    -                                      R                    2                                                  )                                              ]                            -        1            
FIG. 2 shows the system 200 described in the '308 patent. Base 203 supports stage guide 202. Stage guide 202 includes a first rail 282, a second rail 284, and a top portion 286. The stage 204 interfaces with first rail 282 and second rail 284, which provide guidance to stage 204 as it moves along the stage guide 202. The stage 204 also fits closely to the top portion 286, which is expected to aid in reducing variation of distance between a workpiece disposed in fixture 234 and gauges 214, 212. By reducing variation, the stage is expected to increase accuracy and repeatability because changes in amount of extension of plungers (not shown) due to such variations would be reduced, and therefore measurement error and variations between measurements would be reduced.
The stage 204 may be an air bearing stage with a relatively small positioning error and a motion control system that can provide approximately constant velocity. Air bearing stages also help lower error because they tend to distribute load over a large surface area and often have good stiffness which is often desirable for heavy or offset loading. Also, the air bearing of an air bearing stage has an inherent averaging effect that helps in error reduction by filling small surface voids and other irregularities, which is thought to provide better pitch, roll, yaw, and straightness and flatness specifications. An exemplary air bearing stage is the ABL 1000 (FiberGlide 1000) manufactured by Aerotech.
However, there are several drawbacks to the system disclosed in the '308 patent. First of all, since only one sample can be measured at a time, it takes approximately 30 second to measure each sample. Secondly, the system is unable to measure cone straightness. Cone straightness refers to the quality of the sides of the cone. FIG. 3 illustrates some possible undesirable defects in the sides 310 and 315 of a cone 300 that effect cone straightness. Such defects may include a bump 320 as shown in side 310 or a cavity 325 as shown in side 315. Thirdly, the system is sensitive to the effects of particles and other system noise.
Therefore, what is needed is a low-cost, accurate, and repeatable metrology system that is fast, and able to measure cone straightness and cone quality in addition to cone angle.